I am an absolute beginner on OpenBSD, and so I would like to know a couple of basic things.
What graphical desktop environments are available i.e. KDE, Gnome xfce and so on? Where can I get them? And how do I install them?
How do I install the graphics driver in order to enable 3 - D acceleration? And where can I find one? (I am using an Nvidia Geforce 6200 card).
How do I enable sound?
Where can I find all software applications for OpenBSD? And how do I install them?

The OpenBSD project is small. Although you will find a number of sources across the Internet which discuss various aspects of the operating system, the very best source of information on all current aspects of OpenBSD is the official FAQ itself. Many of your questions are answered in this document, & you will save yourself significant aggravation & frustration by studying the entirety of this document now while new to the operating system & culture.

You won't get 3D acceleration for Nvidia Geforce 6200 under OpenBSD and NetBSD. Only FreeBSD supports that.

No FreeBSD doesn't support that! NVidia corporation is the one that released a binary bloob driver for FreeBSD but decided not to release one for OpenBSD and NetBSD due to the small market share.
They are the one who decided to produce closed hardware.

If I recall correctly some of these binary bloob video drivers for FreeBSD can work on NetBSD via some kind emulation. The one in question might not but it is worth further investigation.

No FreeBSD doesn't support that! NVidia corporation is the one that released a binary bloob driver for FreeBSD but decided not to release one for OpenBSD and NetBSD due to the small market share.
They are the one who decided to produce closed hardware.

If I recall correctly some of these binary bloob video drivers for FreeBSD can work on NetBSD via some kind emulation. The one in question might not but it is worth further investigation.

Sjees, is zeebra white in black stripes or black in white stripes?

FreeBSD suports that card if you install Nvidia driver- and then FreeBSD supports it!

Who cares if this is blob or not? That works! I wish I could have such binary blob under NetBSD as I am running it for my desktop computer not super-hiper secure serve- b/c why shall I need Nvidia card for a server??

I do not recall any possibility to run that driver through any emulation under NetBSD- it is just complicated to do that.

So I say yes- FreeBSD as an operating system does support your graphic card Larry.

Who cares if this is blob or not? That works! I wish I could have such binary blob under NetBSD as I am running it for my desktop computer not super-hiper secure serve- b/c why shall I need Nvidia card for a server??

99.9999% of people do not. They run Windows and OS X as they are desktop
operating systems. Who cares what they Internet Service Providers or
Google run on their servers. Apparently NVidia got it right for producing
closed hardware and not releasing drivers for "irrelevant" operating
systems.

So you are trying to tell me that everyone who runs intel wpi3945 wifi card under OpenBSD or NetBSD or FreeBSD doesn't include to thatn 0.0000001 % of users (as we use blobl firmware)? And you also say that FreeBSD is that irrelevant OS as Nvidia driver is available for FreeBSD?

A proprietary driver from some vendor tainting kernel-space is not in any way OK, as the developers cannot reliably debug problems that can and will arise, who knows what it's fiddling with.

OpenBSD does not have binary-blob kernel modules, device firmware in /etc/firmware is totally unrelated.. it is code that is executed by microcontroller located inside hardware, it cannot tamper with critical kernel structures.

I do know that the driver is from Nvidia not from FreeBSD but anyway FreeBSD as a whole supports that card. I admit, blobs are difficult for the developers but allow user to run the device. That's the world.

About firmware, is apart the driver (so you can debug the driver itself I believe) but without that blob firmware you can't use the device anyway- so what's the difference for the end user? Will end user (just a daily user, office or desktop) debug the code?

Oko was right but for the end user only the ability to use the device is important (as long the deiver doesn't crash your system each 5 minutes).